Unity has announced on their blog that they will be deprecating DirectX 9 support in the upcoming Unity 2017.3 release. They explain that reduced DX9 hardware in the market and a lack of support from Microsoft have led to the decision.
Starting with Unity 2017.3, the Unity Editor and Standalone player will no longer support DirectX 9. Unity also recommends developers change the "Graphics API for Windows" setting to use "OpenGL" in order to support the Windows XP platform.
You can learn more at the Unity blog.